296 IVILSO.V. [Vol. IX. 



mass becomes more and more compact until the cells of which 

 it is composed acquire an irregularly polygonal shape owing to 

 mutual pressure (Fig. 25). The ectoderm cells covering the 

 pole become more or less cubical, and at this time do not differ 

 in appearance from the subjacent parenchyma. They neither 

 develop pigment nor cilia, and this end of the embryo is there- 

 fore sharply marivcd off from the rest of the body. The 

 remaining part of the parenchyma is made up of amoeboid 

 cells provided with slender processes connected together into 

 a network. The bodies of all these cells are plump and stain 

 well. When the embryo has reached this stage of development 

 (Fig. 25) spicules make their first appearance. They are few 

 in number and mostly the long, slender oxytylotes. Besides 

 the oxytylotes some curved spicules appear, the embryonic 

 representatives of the bow-shaped spicules shown in Fig. ^d. 



The development proceeds a little farther than the stage 

 shown in Fig. 25, and the embryo is then set free as a ciliated 

 larva which escapes from the body of the parent through an 

 osculum. In PI. XVI, Fig. 26, a surface figure of this larva is 

 given, and in PI. XVI, Fig. 29, a longitudinal section. The 

 greater part of the body is of a deep orange color but the 

 posterior pole (/./.) is unpigmented. The line of separation 

 between the two regions is a perfectly sharp one. The pos- 

 terior pole ends in a pointed protuberance (Fig. 26) which 

 appears to be a specific characteristic. A bundle of straight 

 spicules (oxytylotes) is conspicuous in this end of the larva. 

 In its general appearance and motion the larva is very like 

 a coelenterate planula. Like the latter it may swim freely 

 through the water, or may creep worm-like over the bottom 

 and sides of the dish, the pigmented pole being posterior. 



The cells of the ciliated ectoderm are very long and slender 

 and the nuclei are packed closely in several tiers, so as to form 

 a very conspicuous zone in sections (Fig. 29). The arrange- 

 ment of several ectoderm cells is shown in the maceration 

 preparation, PI. XVI, b. Fig. 31, and one of the cells more highly 

 magnified in a of same figure. There is a single flagellum to 

 each cell. In the peripheral end of the cell is deposited the 

 orange pigment, in the shape of small rounded masses (/.«.)• 



